Low temperature polycrystalline silicon thin film transistors have been widely used in various display apparatuses such as mobile phones, tablets, and watches. A thin film transistor having a high mobility rate may be manufactured by doping the polycrystalline silicon layer. Because polycrystalline silicon has a relatively low resistance, increasing the mobility rate of a thin film transistor often results in a high leakage current, severely affecting electrical properties of the thin film transistor. To decrease leakage current in the thin film transistor, the polycrystalline silicon layer may be made to include a heavy doping region and a light doping region.